Why your downloads are slow, and what to check first

Before checking anything else, check the units. A great many slow download complaints are a connection working exactly as advertised, described in a different unit from the one the download is reporting.

The unit problem

Broadband is sold in megabits per second. Downloads are reported in megabytes per second. There are eight bits in a byte, so the numbers differ by a factor of eight.

A 100 Mbps connection running perfectly delivers about 12.5 MB/s. If your download says 11 MB/s, your connection is doing its job. If you were expecting 100, you were comparing two different units.

The lower case b means bits, the upper case B means bytes. It is a genuinely confusing convention, and it is the first thing to rule out.

Then check the connection itself

Run a speed test while nothing else is downloading, and compare the result with what you pay for. This separates two very different problems: a connection that is underperforming, and a connection that is fine but one particular download being slow.

Test twice if you can, once over Wi-Fi and once with a cable. If the cable is much faster, your broadband is fine and the problem is between your router and your machine.

If Wi-Fi is the bottleneck

This is the most common cause on a home network, and there are four things worth trying in order.

1. Use the 5 GHz band. Most routers broadcast two networks. The 2.4 GHz one reaches further but is much slower and is shared with microwaves, baby monitors and every neighbouring router. If your network names end in something like 5G, connect to that one when you are near enough.
2. Move closer, or move the router. Wi-Fi speed falls off sharply with distance and with every wall in between. A router in a cupboard, on the floor, or behind a television is working against itself. Higher, central and in the open makes a real difference.
3. Check what else is using it. Someone streaming in 4K, a games console updating, or a cloud backup running are all competing for the same connection. Task Manager on your machine shows local network use, and most routers list connected devices.
4. Try a cable for anything large. Even a cheap Ethernet cable removes every variable above. For a 50 GB game download it is worth trailing one across the room temporarily.

When it is the other end

Sometimes your connection is fine and the server is the limit. This is common and there is not much you can do about it, but you can identify it quickly.

The signs are distinctive. One site is slow while everything else is fast. The speed is steady but low, rather than fluctuating. It is slow at particular times of day. Or the download starts fast and settles to a fixed rate, which usually means a deliberate per-user cap.

If the site offers a choice of mirrors or regions, picking one geographically closer often helps considerably. Beyond that, it is a matter of waiting for a quieter moment.

The other usual suspects

A VPN. Everything is routed through another server and encrypted on the way, which costs speed. Halving is not unusual. Turn it off and re-test to see how much it is costing you.

Windows Update in the background. Updates download quietly and can absorb most of your connection. Task Manager, under the Performance tab and then the network view, shows total use so you can see whether something else is busy.

Antivirus scanning as it downloads. Some security software inspects files during transfer, which slows large downloads noticeably. It usually shows up as speed that is fine at first and then drops sharply on big files.

An old router. Wi-Fi standards have moved on considerably. A router more than about six years old may not be able to deliver the speed you now pay for, no matter what the line supports. If your cabled speed is much higher than your wireless speed everywhere in the house, this is likely the cause.

What a download manager does and does not fix

Download managers open several connections to the same file at once. Where a server limits each connection rather than each user, this genuinely helps, sometimes a great deal. Where the limit is your own line, it changes nothing, because the bottleneck is not at the far end.

Their more reliable benefit is resuming. A large download interrupted at 90 per cent normally starts again from nothing in a browser, while a download manager picks up where it stopped. On an unreliable connection that is worth more than any speed gain.

Work through it in this order and you will find the cause quickly: check the units, run a speed test, compare Wi-Fi against a cable, then see whether it is only one site. Four checks, about five minutes, and they separate a genuine problem from a connection quietly doing exactly what it should.